The shadow of Vietnam, and the Vietnam war movie, is never far away during this outstanding fly-on-the-wall documentary about the US military experience in Afghanistan by the British photographer Tim Hetherington and the American journalist Sebastian Junger, famed for his 1997 non-fiction bestseller The Perfect Storm. For one year, Hetherington and Junger accompanied a single platoon on a tour of duty in the dangerous Korangal valley, in which the soldiers – horribly exposed, and with a knowledge of the terrain so far inferior to the enemy's that it was practically blindfold guesswork – had to build a forward outpost to establish their position. This they name Restrepo after one of their popular comrades, Private Juan "Doc" Restrepo, killed at the campaign's outset. This defiant tribute springs from a need to impose their collective identity on this alien and menacing landscape.

The platoon is also required to promote an uneasy hearts-and-minds policy among notionally friendly locals. The soldiers must conduct regular, tense meetings with Afghans, lecturing them about the economic benefits they can bring with supposed transport projects, clearly suspecting every one of them of helping the Taliban, while the locals, though strategically deadpan, clearly resent the Americans. An early sequence shows US military helicopters bringing troops into Afghanistan, then switches to the soldiers' scared, vertiginous point-of-view as they gaze down into the valley in which they will be sitting ducks; it is a very "Nam" moment, and the tension is all but unbearable. You can't help but admire their bravery and that of Hetherington and Junger who have had to keep their nerve and keep filming. The nail-biting sequences are interspersed with interviews conducted after the event, which carry a concealed emotional charge. It is only from these that we can be certain which soldiers have survived.

I was reminded of an essay PJ O'Rourke once wrote about visiting Russia in the 1980s, at the height of the controversy over its invasion of Afghanistan. A Russian teases O'Rourke about his country's recent history: "Vietnam – too bad!" O'Rourke replies crisply: "Land war in Asia very bad – and some countries do not learn from an example!" These soldiers' Asian land war is wearing them down quickly. The Taliban are all around, able to get frighteningly close to the Americans' position, with impunity. Young soldiers are seeing troops being shot dead in front of their eyes. When Junger and Hetherington interview one of them about this experience, he breaks off mid-speech – and of course we, the audience, expect tears: it is a familiar moment in all types of documentary. But what is happening is more disturbing. The man has broken off in a kind of horror at remembering what he has clearly repressed until this moment. It is a flashback – that cinematic term widely applied to post-traumatic disorder. Restrepo is clearly a movie focused on the Americans' fear and suffering, rather than the Afghans', leaving the judgment up to us. It is a scary, moving and troubling film.